The invention relates to a spark-ignition internal combustion engine with direct injection into the combustion chamber which operates according to the jet-guided combustion method, with at least one injection device and one ignition device per cylinder.
In spark-ignition engines with direct injection one distinguishes between wall-guided and jet-guided combustion methods. In wall-guided combustion methods the injection jet is directed more or less directly against the wall of the combustion chamber. This offers an increased preparation time for the mixture formation. As a result of the relatively large freedoms in the arrangement of the injection device, which is mostly close to the cylinder wall, the gas-reversing valves can be designed optimally with respect to position and arrangement. Since injection jet and ignition device are disposed relatively far apart from one another, there is no direct wetting of the spark plug. The system is very insensitive to changes in the shape of the injection jet. The disadvantage is, however, that wall-guided combustion methods have a limited stratification capability. A purposeful wetting of the piston wall is obtained in wall-guided combustion methods. As a result of the longer transport path for the mixture, the basic flow and the fluctuations have a stronger effect on the mixture stratification and thus on combustion.
In jet-guided combustion methods on the other hand, the injection jet is not directed towards the combustion chamber wall or the piston top, but a substantial atomization of the injection jet in the zone of the ignition device is desired. Jet-guided combustion methods have the advantage over wall-guided combustion methods that it is possible to realize an extreme stratification capability which has a direct beneficial influence on fuel consumption. Moreover, a high stability of the stratification can be achieved. The central arrangement of the injection device and the spark plug lead to a reduced size of the gas-reversing valves. A further disadvantage is that a wetting of the ignition device occurs frequently. This can have an unfavorable influence on the combustion stability. The combustion method further shows a high sensitivity with respect to the quality and the shape of the injection jet.
In known jet-guided combustion methods the injection device is usually disposed centrally in the zone of the cylinder axis. The spark plug on the other hand is positioned laterally offset thereto in the combustion chamber ceiling, depending on the constructional conditions. The arrangement of the orifice of the injection device in the zone of the cylinder axis leads to the disadvantage, however, that only very low flow speeds arise in this zone, which leads to the consequence that fuel residues at the orifice are not blown away. This can lead to a coking of the orifice of the injection device, as a result of which the same have a relative short life.
GB 2 276 206 A shows an internal combustion engine with an injection device disposed centrally in the zone of the cylinder axis which is disposed close to a spark plug positioned outside of the center. The injection of the fuel is not performed directly into the combustion chamber, but into an injector chamber of the injection device which is separated from the combustion chamber by a poppet valve.
It is the object of the present invention to increase the stratification capability and the service life of the injection device in a direct-injection spark-ignition internal combustion engine with a jet-guided combustion method.
This occurs in accordance with the invention in such a way that the ignition device is disposed centrally, preferably in the zone of the of the cylinder axis, and the injection device is disposed laterally offset in the direct vicinity of the ignition location, with the distance between the orifice of the injection device and the ignition location being 0.1 to 0.24 times the piston diameter.
As a result of the decentralized arrangement of the injection device, the orifice of the injection device is disposed in a zone of higher flow speeds, as a result of which fuel residues can be blown away effectively. As a result of the ultimate vicinity to the ignition device, it is possible to realize extreme charge stratifications with exceptionally favorable igniting capabilities and, as a result, a particularly low fuel consumption.
It is preferably provided in this respect that the angle of inclination of the jet axis of the injection device to the cylinder head sealing plane is approx. between 40xc2x0 and 80xc2x0 and that the angle between the jet axis of the injection device and the axis of the ignition device is approx. between 10xc2x0 and 65xc2x0, with the axis of the ignition device preferably being disposed parallel to the cylinder axis. A high stratification stability can be achieved by this constructional arrangement. At the same time, the wetting of the spark plug is reduced to a minimum.
In order to form extreme stratifications with high stability it is particularly advantageous to provide in the piston a flat, rounded and substantially circular trough. It is preferably provided that the center of the piston trough, when seen in a plan view, is displaced with respect to the cylinder axis in the direction of the jet axis, with the displacement being particularly preferably between 0.02 to 0.08 times the piston diameter. The piston depth best suited for a lean combustion is obtained between approx. 0.04 to 0.1 times the piston diameter. The mean piston trough diameter should be approximately between 0.35 to 0.5 times the piston diameter.
In order to achieve a particularly high lean-making capability it is advantageous when the combustion chamber ceiling formed by the cylinder head and/or the piston top are shaped in such a way that during the compression stroke of the piston a purposeful flow, preferably a cylindrical flow, is initiated from the injection device to the ignition device at the upper dead center. As a result of the purposeful flow to the ignition device, additional air is guided to the ignition location, thus leading to the prevention of coking of the ignition device. The purposeful flow to the ignition device is promoted when the piston top is provided with a squeezing surface on the side of the injection device.